Nozzle for continuous casting

ABSTRACT

An arrangement and conformation of the discharge openings and channels of a continuous-casting nozzle, together with a specific external profile of the body of the nozzle itself, enable slabs of any thickness, in particular from medium to thin ones, to be cast, which have excellent surface quality and are practically free from inclusions and blowholes.

FIELD OF THE INVENTION

The present invention refers to an improved nozzle for continuouscasting, and more precisely refers to a nozzle suitable for castingslabs, in particular slabs of small and medium thickness, with highcasting rates and improved surface and internal quality of the castslabs.

STATE OF THE ART

As is known, continuous casting of metals and metallic alloys, inparticular steel, consists in transferring, via a refractory materialduct referred to as “nozzle”, the molten metal from a first container,called “tundish”, having the function of distributor and equaliser ofthe flow, into a second bottomless container, called “ingot mould” or“crystallizer”, which is strongly cooled by means of water circulation.At the start of casting, the crystallizer is closed at the bottom by amobile body referred to as “dummy bar”. The molten metal contained inthe crystallizer is protected from oxidation at high temperature bymeans of a layer of lubricating powder, which is continuously renewed.As soon as a sufficient amount of solidified metal has formed inside thecrystallizer, along the walls of the crystallizer and of the dummy bar,the latter is extracted together with the solidified metal in the formof a shell or skin still containing liquid metal. The liquefiedlubricating powder which floats on the molten metal works its waybetween the solidified skin and the walls of the crystallizer, sodiminishing friction. Once outside of the crystallizer, the extractedbody undergoes further cooling, until it is completely solidified, andit is then cut into slabs of convenient length, which are sent on forfurther processing.

Continuous casting has become the casting method most widely used at anindustrial level. This is due to numerous factors, and in particular tothe fact of having available a cast body with a more suitable shape forthe subsequent processes than that of ingots, as well as with atheoretically infinite length, so that it is consequently possible tomarkedly reduce any defects and/or rejects due to segregation, presenceof inclusions, pipes, and the like, which are inherent in the moretraditional ingot casting.

Continuous-casting technology has undergone numerous improvements overtime, in particular linked to the casting rate and to internal andsurface defects of the cast products. This latter aspect is particularlyimportant. In fact, such defects reflect on the surface finish of theend product, which in many cases has to be impeccable, as e.g. forcarbon-steel coils for car bodies, or for stainless-steel coils forarchitectural or aesthetic uses (decorative panels, kitchen-sinksurfaces, cooking surfaces, pots and pans, etc.)), or even on themechanichanical characteristics of the finished product (for example,excessive susceptibility to work hardening; reduced tensile strengthand/or resilience, etc.).

Among the factors affecting the defectiveness of cast products areincluded the thermal, mechanical and fluid-dynamic conditions of theliquid metal in the ingot mould at the level of the initialsolidification of the skin. In fact, the molten metal coming from thenozzle has higher speed and temperature than those of the metal presentin the crystallizer, in which consequently convective currents are setup that can, among other things, draw particles of the supernatantlubricating powders into the body of the liquid metal and up to theviscous zone of start of solidification, with the consequent formationof inclusions, as well as causing sharp differences in temperatureinside the metal such as to induce variations of thickness of thesolidifying skin. A further source of defects is represented by the factthat little circulation of molten metal is possible between the mouth ofthe nozzle and the layer of supernatant lubricating powder, with theresult that the latter may not melt adequately, i.e., in such a way asto guarantee the necessary lubrication between the skin that is formingand the walls of the crystallizer.

The above situation worsens considerably in the markedly expanding fieldof the medium and low thickness slab casting, i.e. slabs having athickness of less than 150 mm, in particular less than 90 mm, where thedisturbance due to entry of the jet of molten metal from the nozzle intothe crystallizer is notably increased.

One of the possible solutions to such problems is to improve thegeometry of nozzles. In fact, nozzles were originally simply rectilinearpipes having the bottom discharging end immersed in the liquid metalpresent in the crystallizer. This structure generated in thecrystallizer strong currents of molten metal directed practically onlydownwards, with irregular recirculation returning upwards along thewalls of the crystallizer. The inadequacy of such a situation was soonrecognized. Consequently, the immersed part of nozzles has undergonenumerous modifications, which basically have involved the creation ofholes with horizontal axes or with axes facing downwards, in the endpart of the nozzle, which has remained essentially tubular. Furthermodifications to the immersed part have subsequently been adopted andhave envisaged a chamber having a cross section greater than that of thenozzle. In this chamber discharging holes have been opened. With theknowledge acquired from such improvements, there has developed anever-increasing awareness of the importance of the formation of patternsof liquid metal flow, as the liquid metal leaves the nozzle, which musthave appropriate shapes, dimensions and rates that may even be differentfrom one another.

Along such a line, the published French patent application FR-A-2 243043 describes a nozzle the end discharging part of which is providedwith a rectangular section distribution chamber with wall parallel tothe walls of the crystallizer, in which the liquid metal coming out ofthe nozzle encounters deflecting walls after a rectilinear path of atleast 100 mm, and is sent on by these deflecting walls towardsdischarging holes with horizontal axes, or else with axes inclineddownwards or upwards. However, the geometry of this nozzle only allows alimited diameter of the discharging holes. Consequently, jets of liquidmetal having high speeds are formed, so maintaining the presence of thedisturbance previously described. Below the nozzle inhomogeneoustemperatures are moreover formed, which adversely affect the quality ofthe cast.

The Italian patent No. 1 267 242 in the name of the present applicantdescribes a nozzle consisting of a discharge duct having a first stretchwith circular cross section which decreases regularly towards a secondstretch, beneath it, with a cross section that varies from circular tobasically that of an elongated rectangle, the lower part of the saidsecond stretch being closed at the bottom by a wall and being providedwith side openings along the shorter sides of the rectangular section.The said openings lead to a chamber which surrounds the bottom part ofthe said second stretch and has holes facing upwards and downwards. Inthis way, the molten metal supplies both the bottom part of thecrystallizer, in which solidification of the metal starts, and the toppart of the crystallizer. Each one of the jets of metal coming out ofthe chamber has a flow rate lower than the flow rate at each of the sideopenings present in the second stretch of the nozzle. In this way, thejets of metal directed downwards cause less disturbance of the thermalflows in the vicinity of the walls of the crystallizer, thus renderingthe thickness of the skin that is forming more constant, whilst the jetsdirected upwards favour maintenance of high temperatures in the top partof the crystallizer, thereby ensuring complete melting of thelubricating powder used for protecting the molten metal and preventingthe formation of “cold” spots, at which there could occur an undesirablesolidification of the metal.

Experience has shown, however, that, albeit representing an improvementover previous nozzles, a nozzle having the above structure is, on theone hand, suitable only for continuous casting of thin slabs, whereas onthe other it does not achieve completely the advantages set forth in thedescription. In particular, the problem remains, which is moreovercommon to all nozzles, of the poor feed of molten metal upwards in theregion around the descending duct of the nozzle. In this region, thevicinity of the cooled walls of the crystallizer to the nozzle, combinedwith a poor circulation of the molten metal coming directly from thenozzle, and hence at maximum temperature, easily causes the formation ofcold spots. In addition, the relatively low temperature around thenozzle may lead to the failure of the supernatant lubricating powder tomelt in situ, with possible drawing along of solid particles oflubricating powder in the solidification zone.

SUMMARY OF THE INVENTION

The aim of the present invention is to overcome the drawbacks referredto above by proposing a nozzle for continuous casting of slabspreferably having a thickness of between 40 and 200 mm and a width ofbetween 700 and 3200 mm. This purpose is achieved by the design of anozzle which provides a plurality of discharging channels directeddownwards and upwards, part of the channels directed upwards havingwalls with a winged profile; in addition, the section of said nozzle isappropriately variable in a continuous fashion. In this way, areobtained a first flow of liquid metal upwardly coming out of,the nozzle,said first flow lapping the descending duct of the nozzle itself, aswell as a second flow upwardly directed towards the regions closest tothe smaller walls of the crystallizer, and also a third low speed flowof liquid metal downwardly directed in such a way as to involvepractically the entire section of the crystallizer.

With a number, configuration and arrangement of discharge channels ofthis sort, the upwardly directed flows of liquid metal have a low speedand are distributed uniformly over the entire section of thecrystallizer, thus ensuring: (i) a good uniformity of temperature of theliquid metal at the level of the meniscus; (ii) a complete liquefactionof the lubricating powder; and (iii) the absence of vortices at thelevel of the meniscus, which might determine trapping of the lubricatingpowder.

On the other hand, also the downwards directed flows are uniform andrelatively “gentle”, so enabling any possible gas bubbles and inclusionsdrawn along by the liquid metal to return back up towards the meniscus.In addition, the direct impact of the jet of liquid metal against theskin that is solidifying is prevented, so eliminating, or at leastmarkedly reducing, the so-called “washing” phenomenon.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described in detail with reference tothe attached drawings, which show possible embodiments of the inventionand in which:

FIG. 1 is a longitudinal sectional view of a first embodiment of anozzle according to the invention;

FIG. 2 is a longitudinal sectional view of a second embodiment of anozzle according to the invention;

FIG. 3 is a longitudinal sectional view of the nozzle of FIG. 1,according to a plane orthogonal to the plane of FIG. 1;

FIGS. 3 a–3 e each show a cross section of the nozzle of FIG. 3.

In the figures, similar parts are identified by the same referencenumbers; in addition, for reasons of simplicity, in one and the samefigure with specular parts, some reference numbers are indicated in oneof the parts and other reference numbers in the other. Finally, some ofthe reference numbers indicated in one figure may not be indicated inanother figure, in order to prevent any reading mistakes. However, it isunderstood that the said reference numbers and indications are valid forall the similar figures.

The nozzle according to the present invention is used for continuousfeeding a liquid metal into a crystallizer for the continuous casting ofslabs, preferably having a medium to small thickness, in which, in fulloperating conditions, a metal bath provided with a free surface referredto as meniscus, generally covered with lubricating covering powders, ispresent, and from which a body is continuously extracted, which is madeup of a solidified skin still containing some solidifying metal. Thenozzle is made up of an elongated tubular body 11 made of refractorymaterial having a first top part 11 a of a roughly cross section, and asecond bottom part 11 b, which is radiused to the first part and has aflattened cross section and roughly pointed end regions 11 c, and ispartially immersed in the metal bath and has, at the bottom, in eachroughly pointed end region, a discharging hole 13 a, 13 b, the saidsecond part further having, in its bottom end part, beneath the saiddischarging holes; a closing wall 12, which may be flat (FIG. 1), orelse provided with a cusp 24 facing towards the inside of the nozzle(FIG. 2). Each of the said holes, which face one another, gives out intoa laterally elongated chamber 14 a, 14 b, which is in turn provided withholes 20, 21, 22 to enable passage of liquid metal from the nozzleitself towards the inside of the crystallizer. The said bottom part 11 bof the tubular body 11 made of refractory material may have a flattenedpolygonal cross section with rounded edges, or else an ellipticalsection, with opposite ends 11 c that are roughly pointed, and each ofsaid elongated chambers 14 a, 14 b, each defined by two larger walls 14c, 14 c′ and by deflecting elements 18, 19, is equipped with at leastthree discharging doors 20, 21, 22 designed to divide and distribute thejet of molten metal according to at least three preferential directionson each side of the nozzle, by means of said respective deflectingelements. In each of said chambers, at least two of the dischargingdoors are set facing upwards, and at least one of the discharging doorsis set facing downwards, one of the doors facing upwards being adjacentto the said second bottom part of the tubular body and partiallysurrounding the pointed or edge-shaped end region 11 c thereof, asillustrated in FIG. 3 d.

In this way, preferential currents of molten metal are created, directedupwards and downwards. The doors 20 adjacent to the bottom part of thetubular body each have the shape of a duct with the longitudinal axis 15preferably parallel or convergent upwards with respect to thelongitudinal axis 11 e of the nozzle 11, and with a face 181 having awinged profile with its concavity facing said tubular body. The endparts, bottom and top, of said face with winged profile form,respectively, leading angles β2 and trailing angles β3, with respect tothe axis 11 e of the nozzle, preferably between 0° and 45°, it beingpossible for said angles β2 and β3 to be equal to one another.

In this way, an upwardly directed metal jet is created which laps theouter walls of the nozzle along said edge 11 c and which sends a jet ofmetal into the part of meniscus around the nozzle itself such as toguarantee uniformity of temperature with respect to the other regions ofthe meniscus.

At least one of said discharging doors facing upwards has the shape of aduct with a cross section that increases from the inside towards theoutside, with a longitudinal axis diverging, by an angle β1 of between10 and 80°, upwards with respect to the longitudinal axis of saidelongated tubular body. In this way, a jet of liquid metal is generateddirected towards the narrower walls of the crystallizer.

The combined action of the said upwardly directed jets of liquid metalsupplies the top part of the bath present in the crystallizer, and henceits meniscus, in a considerably uniform manner, such as to maintain theentire region of the meniscus suitably hot, and so creating the idealconditions for melting of the lubricating powder in order to diminishfriction in the ingot mould, the said jets having, in any case, arelatively low speed, in such a way as to disturb as little as possiblethe flow of liquid metal circulating in the top part of thecrystallizer.

Preferably, the deflecting elements 18, 19, which direct the jets ofmetal in the desired directions, constitute the elements of separationbetween contiguous discharging doors.

The said elongated tubular body 11 has a first stretch 11 a with asection of constant area, and a second, lower, stretch, 11 b having asection that increases in the direction of the said chambers 14 a and 14b for distributing and discharging the metal. Preferably, the said firststretch 11 a has a section of a circular type (FIG. 3 a), whilst thesecond stretch 11 b has a section that varies continuously fromcircular, at the point where it joins with the said first stretch (FIG.3 b), to an elongated flattened profile (FIG. 3 d) in the vicinity ofthe said distributing and discharging chambers, it being possible forthe said flattened profile to be, for instance, octagonal or elliptical.

In a preferred embodiment, the distance between the internal wallsmeasured along the major internal axis D3, and the distance measuredalong the minor internal axis D2 of the section of the end part of thesaid second stretch are, respectively, greater and smaller than theinternal diameter of the circular section. The angles α1 and α2 betweenthe longitudinal axis of the nozzle and, respectively, the edge of saidpointed end region of the flattened part of the nozzle and the face orregion at 90° from the said edge, are, respectively preferentiallybetween 2° and 8° and between 0° and 4°.

An essential aspect of the invention is that flows of metal havingspeeds and flow rates suited to the attainment of the requiredperformance in terms of reduction in internal and surface defects andincrease in plant output must be created.

For this purpose, the sections of the various passages present areashaving appropriate ratios to each other.

In particular, the said second, bottom, tubular part 11 b of the nozzlehas a ratio between the internal area A01, at the level of the saiddistributing and discharging chambers, and the internal area A0, at thelevel of the join with said first top part, of between 1.1 and 1.7.

In addition, the ratio between the exit area A1 of each of the topdischarging doors adjacent to the said second bottom part of the nozzleand the said area A01 is between 0.15 and 0.35, whilst the ratio betweenthe exit area A2 of the other discharging doors facing upwards and thesaid area A01 is between 0.20 and 0.40.

As far as the doors facing downwards are concerned, for these the ratiobetween the exit area A3 and the said area A01 is between 0.15 and 0.75.

1. A nozzle for continuously feeding liquid metal into a crystallizerfor continuous casting of slabs, the nozzle comprising: a refractoryelongated tubular body having: a first top part having a substantiallycircular cross section; and a second bottom part, radiused to the firsttop part, having: a substantially flattened section provided at a bottomportion thereof with lateral discharging holes; and a closing wall in abottom end part thereof beneath the discharging holes, having a shapedselected from the group of flat and cusp-shaped facing the inside of thenozzle, wherein each of the discharge holes is set facing one another,opening out respectively into a laterally elongated chamber, in turnprovided with channels for enabling passage of the liquid metal from thenozzle itself towards the inside of the crystallizer; and deflectingelements, wherein each of the elongated chambers is equipped with atleast three discharging doors designed to divide and distribute a jet ofliquid metal according to at least three preferential directions on eachside of the nozzle, by the respective deflecting elements; wherein twoof the discharging doors on each side of the nozzle face upwards; andwherein at least one of the upwardly facing discharging doors on eachside of the nozzle has the shape of a duct with a cross-section whichincreases from the inside outwards, with a longitudinal axis upwardlydiverging by an angle β₁ between about 10° and about 80°, with respectto a longitudinal axis of the elongated tubular body.
 2. The nozzleaccording to claim 1, wherein a bottom part of the elongated tubularbody has a cross section selected from the group of elliptical andflattened round-edged polygonal profile with substantially lateralfacing ends.
 3. The nozzle according to claim 2, wherein angles α1 andα2 between the longitudinal axis of the nozzle and, respectively, anedge of a pointed end region of the flattened part of the nozzle and theface or region at about 90° from the edge are, respectively, within thepreferential range of about 2° to about 8° and about 0° to about 4°,respectively.
 4. The nozzle according to claim 1, wherein each of theelongated chambers is defined by two larger walls and by the deflectingelements.
 5. The nozzle according to claim 1, wherein one of the upwardsfacing discharging doors on each side is adjacent to the bottom secondpart of the tubular body and partially surrounds the pointed or edgedend region thereof.
 6. The nozzle according to claim 1, wherein at leastone of the discharging doors on each side is facing downwards.
 7. Thenozzle according to claim 1, wherein the doors adjacent to the bottompart of the tubular body each have the shape of a duct with longitudinalaxis parallel or convergent, upwardly, to longitudinal axis of thenozzle, and with a winged profile face having a concavity facing thetubular body, the bottom and top end-parts of the face having,respectively, leading angles β2 and trailing angles β3 being betweenabout 0° and about 45°.
 8. The nozzle according to claim 7, wherein theangles β2, β3 are equal to one another.
 9. The nozzle according to claim1, wherein the elongated tubular body has a first stretch with a sectionof constant area, and a lower second stretch having a section whichincreases in the direction of the chambers for distributing anddischarging the liquid metal.
 10. The nozzle according to claim 9,wherein the first stretch has a circular section, and the second stretchhas a continuously variable section, beginning from a circular profile,at the join with the first stretch, to an elongated flattened profile,in the vicinity of the distributing and discharging chambers, with theflattened profile being polygonal.
 11. The nozzle according to claim 9,wherein the first stretch has a circular section, while the secondstretch has a continuously variable section, beginning from a circularprofile, at the join with the first stretch, to an elongated flattenedprofile, in the vicinity of the distributing and discharging chambers,with the flattened profile being elliptical.
 12. The nozzle according toclaim 11, wherein the distance between the internal walls measured alongthe major internal axis, and the distance measured along the minorinternal axis of the section of the end part of the second stretch are,respectively, greater and smaller than the internal diameter of thecircular section.
 13. The nozzle according to claim 1, wherein thedeflecting elements, which direct the jets of liquid metal in thedesired directions, include regions of the refractory walls of thechambers and constitute a separation between contiguous dischargingdoors.
 14. The nozzle according to claim 1, wherein the second tubularbottom part of the nozzle has a ratio between a first internal area atthe level of the distributing and discharging chambers and a secondinternal area at the level of the joint with the first top part betweenabout 1.1 and about 1.7.
 15. The nozzle according to claim 1, whereinthe ratio between the first exit area of each one of the top dischargingdoors adjacent to the second bottom part of the nozzle and apredetermined area is between about 0.15 and about 0.35, and the ratiobetween the second exit area of the first discharging doors facingupwards and the predetermined area is between about 0.20 and about 0.40.16. The nozzle according to claim 1, wherein the ratio between the thirdexit area of the second doors facing downwards and the predeterminedarea is between about 0.15 and about 0.75.